The production of “What Ever Happened to Baby Jane” has come and gone on FX’s “Feud,” but as the latest episode showed, that was hardly the end of the beef between Joan Crawford (Jessica Lange) and Bette Davis (Susan Sarandon).

Sunday night’s installment told the lurid, behind-the-scenes story of the production on their follow-up picture, “Hush . . . Hush, Sweet Charlotte,” which was held up for several weeks when Crawford took a “sick leave” from the film. It went on so long that 20th Century Fox brought in its lawyers, who offered to release Crawford from her contract. When she said no, they demanded she submit to an independent medical exam, as the studio’s insurance company was not going to pay for a fake illness. The lawyers were very clear with Crawford: If the doctor pronounced her fit to return to work and she did not, the studio would sue her.

“When Joan checked herself into Cedars-Sinai, it was like checking into the Beverly Hilton. She had food catered from Chasen’s, and that really drove Bette up the wall,” he told The Post. “She thought it was cowardly. Joan knew she didn’t have the acting chops [to do the part]. What was a rivalry before became an all-out war.”

Director Robert Aldrich’s (played by Alfred Molina in the series) daughter Adell, a script supervisor on the film, shared her insights in the documentary “Hush . . . Hush Sweet Joan”: “Did she become ill? Dunno,” she says. “Production shut for a period of several weeks, and in that time my father was given a list of women to replace her with.”

Crawford’s grave miscalculation that her star power could shut down the picture became woefully apparent when Aldrich, at Davis’ behest, flew to Olivia de Havilland’s (Catherine Zeta-Jones) chalet in Switzerland to sweet-talk her into playing Crawford’s role. Having already starred in the overwrought “Lady in a Cage,” a part Crawford turned down, de Havilland was reluctant to play Charlotte’s evil cousin Miriam, but Aldrich convinced her she could approach the role in a more understated fashion.

With Crawford out of the way, “Hush . . . Hush, Sweet Charlotte” resumed production — this time without the behind-the-scenes drama. Longtime friends Davis and de Havilland respected each other as colleagues. “It became a wonderful film to be part of,” Adell said in the documentary.

A box-office success, “Hush . . . Hush, Sweet Charlotte” was nominated for seven Oscars, including Best Supporting Actress for Agnes Moorehead, who won the Golden Globe that year.